1. Field of the Invention
The invention relates to a fingerprint sensor, and more particularly, to an optical fingerprint sensor with variable resistors.
2. Description of the Prior Art
Please refer to FIG. 1, FIG. 1 shows a functional diagram of conventional fingerprint sensor 10. The fingerprint sensor 10 is a capacitive fingerprint sensor that senses fingerprints by detecting changes of capacitances. The fingerprint sensor 10 comprises a detecting and processing circuit 12 and a sensing area 14. When a user fingers the sensing area 14, the related circuits of sensing area 14 produce corresponding signals in accordance with the fingerprint and transfer the signals to the detecting and processing circuit 12 to analyze and identify.
FIG. 2 is a circuit diagram of the sensing area 14 in FIG. 1. The sensing area 14 comprises a plurality of sensing units 16 arranged in matrix. Each of the sensing units 16 is used for sensing the texture of a fingerprint on the corresponding position and comprises a transistor 18 and a detecting capacitor Cf. When the sensing area 14 acts, the source S of the transistor 18 is given a detecting signal Vs1 or Vs2. When the user fingers the surface of the sensing area 14, the capacitance of the detecting capacitor Cf is changed. As the capacitance of the detecting capacitor Cf is changed, the gate voltage of the transistor 18 is also changed with capacitive coupling effect. The variation13Δ Vg of the gate voltage of the transistor 18 is decided by the capacitance of the detecting capacitor Cf and is showed as:ΔVg=Vs·Cgs/(Cgs+Cf)where Vs is the voltage value of the detecting signal Vs1 or Vs2;
Cgs is the parasitical capacitance between gate G and source S of the transistor 18; and
Cf is the capacitance of the detecting capacitor.
The variation_Δ Vg of the gate voltage of the transistor 18 directly influences the electric current I which flows through the transistor 18. The detecting and processing circuit 12 identifies the fingerprint in accordance with the variation of electric current I. However, detecting the variation of electric current I to identify the user's fingerprint is usually interfered with by the leakage current of the adjacent transistor 18 and limits the accuracy.